1. Field of the Invention
The present invention relates to a memory system.
2. Description of the Prior Art
Recently, video memory capacities have remarkably increased, as the resolving power of screens have been enhanced and also as the number of colors to be displayed has increased.
In general, when video memory capacity is expanded a problem arises in that the time required to process data to be displayed is extended in proportion to the capacity increase. To solve this problem, conventional display data processors have been improved to speed up the time to process display data. However, there is a limit to memory access speed.
In other words, when a display memory is of a word-structure, the time for accessing each pixel will be unfavorably delayed. Also, when the display memory has a side-by-side-pixel structure, another problem arises in that bit locations must be processed within a word, resulting in delayed access.
In particular, the problem associated with a display memory having a word structure will be described below. In connection with this description, FIGS. 7(A)-(D) provide an explanatory schematic of a word structure display memory.
When character information is to be written into a word structure display memory, it can be expected that the value of a plane select register is set according to a color code and font pattern data is written (see FIG. 7 (A)). In this case, the character information can be written at very high speed.
However, if the background color is not "0", then the information or data must be written into respective planes separately. Therefore, the greater the number of planes to write, the slower the writing speed is (that is, the writing operation requires an amount of time proportional to the number of planes).
Also, when only 1 dot of a display screen is to be written a longer time is necessary for processing it. This is because after data is read out from each of the planes (FIG. 7 (B)), only one or more bit locations to be altered are written back to "1" or "0" (FIG. 7 (C)), which operation must be performed repetitively for all of the planes (FIG. 7 (D)).
In general, when display data is written, most of the images/pixels are formed by combining dot-by-dot writing operations and thus the writing operations require a very long time for processing.
On the other hand, in the case of a display memory having a side-by-side-pixel structure another problem exists as follows. In connection with this description, FIG. 8 provides an explanatory schematic of a side-by-side-pixel structure display memory.
When writing display information into the display memory, a font pattern must be extended to a display color and a background color before it is written, which makes the writing time longer. Also, when the pixel length is long, the number of pixels which can be written at a time is reduced so that it takes a still longer time to write the display information.
In the side-by-side-pixel structure, it is easier to perform a dot-by-dot writing operation than in the above-mentioned word structure, but bit locations within a word vary according to pixel positions. For this reason, in order to update a pixel, a CPU or a video data processor must read out a word in which the pixel exists, then shift the pixel to the position of the bit 0 and then process it, wherever the pixel to be updated is located, next return the pixel to its prior-to-shift location, and write it back after adjacent data is added thereto.
Thus, the word structure display memory and the side-by-side-pixel structure display memory both have problems, and memory access time is a considerable limitation to overcome before performance can be enhanced.
Also, in processing an image, when the image boundary is detected or delineated (that is, an area within the boundary is given a predetermined color), it is necessary to detect a color code which is written in an image memory. In this case, data is read out from the above-mentioned image memory, a color code for each pixel is extracted from the data, and the extracted color code is compared with a desired color code in pixels.
Whether the image memory employs the word structure or the pixel structure, in order to compare color codes, it is necessary to extract a color code for each desired pixel. This means that it takes a long time to compare and detect the above-mentioned color codes. This problem remains unsolved in the prior art.
Further, the conventional memory unit is only capable of storing a given piece of information and simply reading and writing the information, that is it has only a single function. Thanks to the progress of semiconductor technology, however, a multitude of possible functions can be realized in addition to the single read and write function.
The conventional memory unit capable of only the single read and write function is becoming less and less valuable. This is a problem in that suppliers of obsolete equipment will realize decreased profits.
The conventional memory unit has also been increased gradually in capacity. So there is a tendency to expand its additional circuits.
On the other hand the memory unit of an image device can be used in two ways, that is it can be used as a video memory or as a program memory. The video memory has its own access method, while the program memory has its own access method. These two access methods are different from each other.
Therefore, a data processing system which stores large files of image data requires a video memory of large capacity and a program memory of large capacity. At the same time, the peripheral circuits of these memories must also be large-sized so that the whole system, disadvantageously, must be large-sized.